1. Field of the Invention
The present invention relates to a two-shaft gas turbine equipped with a gas generator and a power turbine and structured so that compressed air flows back to a combustor via outside the gas turbine. More particularly, the invention relates to gas turbine fuel control to ensure stable fuel combustion at start-up of the gas turbine.
2. Description of the Related Art
A typical two-shaft gas turbine includes a gas generator that includes a compressor, a combustor having multiple fuel systems, and a high-pressure turbine coupled coaxially with the compressor; and a power turbine (low-pressure turbine) coupled with a load (e.g., generator), the rotating shaft of the gas generator being separated from that of the power turbine. With this type of gas turbine, the rotating speed of the high-pressure turbine and that of the low-pressure turbine can be controlled independently of each other.
Japanese Patent No. 4972533 discloses a one-shaft gas turbine furnished with a humidifier and a recuperator as equipment external to the gas turbine, as well as multiple fuel systems. This one-shaft gas turbine is equipped with an air extraction channel for supplying compressed air to the humidifier and an injection flow channel for injecting air coming from the recuperator into the combustor. The cited Japanese patent further discloses a method by which, when combustion air varies in temperature depending on the temperature state of the recuperator, the fuel systems are switched (i.e., the number of fuel systems to be ignited is selectively increased or decreased) on the basis of the relationship between electric power generation and combustion air temperature so that gas turbine operability and fuel stability are both ensured.
Furthermore, Japanese Patent No. 4464226 discloses a control method by which, when combustion air temperature varies in a plant equipped with a one-shaft gas turbine having a humidifying tower and a recuperator as external equipment, output auxiliary control factors are corrected on the basis of combustion air temperature so that the plant may be started up quickly and stably.